I have long used custom tethers, because the bow of a cat is too wide for 6' to make sense. Making your own requires a good knowledge of sewing and strength. But small boat sailors often need shorter tethers, and they would be happier not sewing there own.

How would you shorten a tether?

Tie-off a loop. Messy, weakens it a bit, but almost certainly not enough to matter.

Fold flat and sew. You could add a gusset and make it full strength, or you could assume the stitching would tear out above 500 pounds, providing shock absorption while getting longer.